This invention relates to flexible foams and flexible molded foams prepared from an isocyanate component comprising an allophanate-modified diphenylmethane diisocyanate. The present invention also relates to processes for the production of these flexible foams and flexible molded foams.
Flexible polyurethane foams and processes for their preparation are known and are described in, for example, U.S. Pat. Nos. 4,478,960, 4,833,176, 4,876,292, 4,945,117, 5,070,114, 5,369,138, 5,521,225 and 5,521,226, and in European Patents 0010850 and 0022617. Mixtures of diphenylmethane diisocyanate (MDI), poly(phenylmethane isocyanate), prepolymers based on such materials, and toluene diisocyanate are known to be suitable for preparing flexible foams. See, for example, U.S. Pat. Nos. 4,239,856, 4,256,849, 4,261,852 and 4,365,025.
U.S. Pat. No. 5,070,114 discloses the preparation of flexible foams prepared from diphenylmethane diisocyanate (MDI) based prepolymers having a rather low NCO value and from an isocyanate-reactive composition comprising relatively high amounts of water.
U.S. Pat. No. 4,478,960 describes the preparation of flexible polyurethane from 1) a prepolymer based on MDI and optionally polymeric MDI, and a polyol having from 5 to 30% by wt. of oxyethylene groups, wherein the prepolymer has an NCO group content of 12 to 30% by wt., 2) polymeric MDI, if 1) above is MDI, 3) a polyol having a low EO content and 4) a blowing agent.
A process for the preparation of a flexible foam from a prepolymer having an NCO content of from 15 to 30% by wt. is disclosed by U.S. Pat. No. 4,945,117. These flexible foams are prepared from a polyol having a functionality of at least 4.
The preparation of flexible foams from polyisocyanate prepolymers is disclosed by U.S. Pat. No. 5,369,138. The prepolymer is reacted with water and a mixture of a polyol having a low oxyethylene content and a polyol having a high oxyethylene content.
U.S. Pat. No. 4,876,292 describes a particular isocyanate-reactive mixture suitable for preparing flexible polyurethane foams. This mixture comprises up to 3 parts by wt. of an amine corresponding to a specific formula, up to 30% by wt. of a polyoxyalkylene polyamine having a molecular weight of from about 400 to about 5000 and containing from 2 to 3 primary amino groups, and from 70 to 100% by wt. of one or more polyether polyhydroxyl compounds having hydroxyl functionalities of from 2 to 3 and molecular weights of from 1000 to 10,000. Polymethylene poly(phenyl isocyanates) or prepolymers thereof are suitable for reaction with these isocyanate-reactive mixtures.
The process for producing flexible foams disclosed by U.S. Pat. No. 5,521,225 comprises reacting a polyisocyanate composition having an NCO group content of 10 to 25% by wt., with a specific polyol composition. Suitable polyisocyanate compositions comprise a) an isocyanate prepolymer having an NCO content of 5 to 15% by wt. and being prepared by reacting an excess of polyisocyanate and a polyol of specified functionality, equivalent weight and EO content, and b) a polyisocyanate having an NCO content of 30 to 33% by wt.
Allophanate modified isocyanates are also known in the art. Various isocyanates containing allophanate groups and processes for their production are disclosed in, for example, U.S. Pat. Nos. 4,738,991, 4,866,103, 5,319,053 and 5,319,054, European Patents 0,031,650 and 0,393,903, and GB Patent 994,890. European Patent 0,393,903 is of relevance; it discloses some examples wherein flexible foams are prepared. These flexible foams, however, are produced from an allophanate-modified isocyanate based on MDI containing 20% by weight of the 2,4'-isomer of MDI, and high molecular weight polyether polyols having a functionality of 2 to 3.
U.S. Pat. No. 4,738,991 is also of interest in that it discloses one example wherein a flexible foam is prepared. This flexible foam, however, is produced from an allophanate-modified isocyanate based on toluene diisocyanate which is prepared according to the process described therein.
European Patent 0,031,650 describes MDI based allophanate modified isocyanates. These blends, however, consist of an allophanate modified MDI which is the reaction product of an alcohol with an MDI based isocyanate comprising more than 20% by weight of the 2,4'-isomer of MDI. It also discloses that these may be suitable isocyanates for flexible foam molding.
It has presently been found that allophanate modified diphenylmethane diisocyanates can be used to produce flexible foams and flexible molded foams which exhibit excellent properties. More specifically, flexible foams and flexible molded foams produced from these allophanate modified diisocyanates exhibit increased tear strengths, tensile strengths and elongations with sharp end cure and reactivity profiles. When compared to conventional and/or commercial isocyanates which are typically used in flexible foams and flexible molded foams, these allophanate modified diphenylmethane diisocyanates result in foams having tear strengths which are 4 to 5 times higher and elongations which are 50 to 100% greater.
It was also noted that the flexible foams and flexible molded foams produced from these allophanate modified diphenylmethane diisocyanates have high compression sets. However, the improved strength properties make these foams very attractive for specialty applications including, for example, packaging, toys, specialized medical seating and archery targets.
Another advantage offered by the allophanate modified diphenylmethane diisocyanates is an increased processing window. Conventional polymeric MDI based systems have a processing window between 115.degree. to 140.degree. F. Below 115.degree. F. mold temperatures, the systems are sensitive to mold temperature tending to form a "skin" on the surface and form pressure voids. Allophanate modified isocyanates extend the processing window down to at least about 100.degree. F., with no loss in the other processing requirements. This attribute provides significant commercial advantage, offering a robust system which is less prone to problems due to mold temperature variations.
It was also found that compression sets of these foams could be lowered by blending the allophanate modified diphenylmethane diisocyanates with polymethylene poly(phenyl isocyanates), i.e., PMDI. While blending PMDI will lower compression sets, we found that the addition of an amine based alcohol crosslinker, such as, for example, diethanolamine (DEOA) further drops the compression set into the area where they would be useful as automotive flexible foams in, for example, headrests, armrests, and seating.